An arrangement of the kind described above is disclosed in U.S. Pat. No. 4,441,471. The arrangement for controlling the idle engine speed disclosed in this patent includes a desired-value forming unit which forms an engine speed desired value in dependence upon various operating variables of the engine and of the motor vehicle. This desired value is compared to the actual measured engine speed value in a comparator unit. A controller forms an output signal in dependence upon the difference between the desired and actual values of the engine speed. The output signal is provided for adjusting the actuator element influencing the speed of the engine in the sense of reducing the difference between the desired and actual engine speeds. A transmission control signal is applied for forming the desired value in order to avoid jumps in the engine speed when a switch-over is made from the neutral position to the drive position in an automatic transmission.
The engine speed fluctuates as a consequence of the compression and combustion during operation of the engine. The amplitude of these fluctuations increases with decreasing engine speed. These engine speed fluctuations can excite the vehicle drive and/or the vehicle itself into oscillations especially when the exciting frequency lies close to a resonant frequency of the engine drive and/or of the vehicle itself. Experience has shown that such oscillations (bucking) take place especially at low engine speeds in the range of idle engine speed in the presence of a force connection between engine and transmission which oscillations operate to reduce the driving comfort in this operating condition. This becomes especially manifest when driving in a column or in heavy traffic when the motor vehicle is moving forward at lower engine speed in the range of idle engine speed and in the presence of a force connection, that is, for example in the case of a manual gear shift with a gear in place and the clutch engaged.